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Old 3rd April 2012, 01:45 PM   #1
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Default simple se input impedance

I have a capacitor in series with the input of my simple se. This way the amp only amplifies 250 Hz and up which is sent to my Full range open baffle speakers. The bass is a H frame powered by a different amplifier (Moscode )
The smallest high end capacitor I can find (Parts express is a Kimber kap .0047uF.
This gives a high pass with this amp of 250 Hz. (Thats the effect of the speaker. There may be a slight rise caused by the speaker changing the "effective" crossover.
Is it possible to lower the input impedance of the amp to change this effect?
I really only want to change the crossover effect to 175 Hz -200 Hz.
Thanks
Paul
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Old 3rd April 2012, 02:16 PM   #2
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The input impedance at audio frequencies is mainly defined by grid resistor, so you can down its value as you want, but never exceed the value recommended by the manufacturer limit by grid current and emission. So the cut off frequency will be :

f in [Hz] = 1/(2 pi [ohm] C[farad])

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Old 3rd April 2012, 02:35 PM   #3
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The only consideration in reducing the grid resistor is whether the preamp can drive it. Any competent pre should easily drive anything over 10K.
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Old 3rd April 2012, 02:53 PM   #4
DF96 is offline DF96  England
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To lower the LF point with a given coupling cap you need to raise the grid resistor, so you do need to be aware of any upper limit.

Why worry about 'high end' caps when the SE will generate distortion anyway?
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Old 4th April 2012, 03:01 AM   #5
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Why are tube amp input impedances usually in the 200k range and SS amps in the 50 K range?
So I can just reduce the 220K resistor to ~150K
we are talking about the resistor between input and ground? R11 and R21?
Simple SE schematic

Thanks
Paul
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Old 4th April 2012, 04:15 AM   #6
chrish is offline chrish  Australia
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You do realise that a simple r/c filter will only give you a cutoff slope of 6dB/octave? This will be in addition to any rolloff of the speaker/box combination. You will also have to consider the cutoff slope of the bass driver/box/crossover combination.

Active crossovers may be a better option Linkwitz-Riley Electronic Crossover
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Old 4th April 2012, 09:38 AM   #7
DF96 is offline DF96  England
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Quote:
Originally Posted by pforeman
Why are tube amp input impedances usually in the 200k range and SS amps in the 50 K range?
Because valve grid current is generally much smaller than BJT base current, so circuit impedances can be higher.
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Old 4th April 2012, 01:45 PM   #8
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Quote:
Originally Posted by chrish View Post
You do realise that a simple r/c filter will only give you a cutoff slope of 6dB/octave? This will be in addition to any rolloff of the speaker/box combination. You will also have to consider the cutoff slope of the bass driver/box/crossover combination.

Active crossovers may be a better option Linkwitz-Riley Electronic Crossover
Am using a active crossover on the bass units. Simple 6dB/ octave is fine for me on the full rangers. I have no problem with integration of the two units and plenty of headroom.
After reading lots of posts where people claim problems with the H frame crossed over any higher than ~70 Hz, I thought I'd see if perhaps with a simple resistor change I might like the sound even more!
In my room with my Audio Nirvana 10" open baffles, I have a slight excess of sound in the 200 -400 Hz range (about 3dB ). I've even thought about raising the crossover point a little to help reduce this? Or maybe eliminating the sides on the open baffles?
So far it sounds good enough that I'm not in a hurry, but because I'm out of projects, I'll probably try something.
Changing the input impedance sounds like the easiest if its possible.
Trouble is that in these tiny values there aren't that many values available.
Paul
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